This invention relates to the process and apparatus for measuring the resistance to flow of a patient's blood under conditions approximating the microcirculatory vessels. Resistance to flow of blood is measured as an apparent viscosity, the flow taking place through a porous bed. The apparent viscosity of blood decreases from indefinitely large values near zero flow rates to an asymptotic value of the order of 3 to 5 centipoise over wall shear stresses of the order of 5 dyn/cm.sup.2.
It is important to provide a screening test for large patient populations to determine a patient's blood shear-rate dependent viscosity in order to determine whether further analysis is required to measure the factors in the blood that affect blood viscosity. The non-Newtonian behavior of blood viscosity is determined by hematocrit (red cell volume percent) and macromolecular concentrations, primarily fibrinogen which in turn determines the ease of blood flow through microvasculature of the body and this varies widely from patient to patient. When blood flow is reduced during the normal course of passive and active distribution control, red blood cells and fibrinogen act to form red blood cell clusters or rouleaux which can cause undesirable stoppage of microvascular flow. Rouleaux require certain levels of fluid shear stress to cause their breakup. When the blood's fibrinogen is high, fluid shear stress required to break up the rouleaux or to restart microvascular stoppage is commensurately higher and sufficient energy may not be available from the normal proximal flow. These aggregate phenomena can aggravate or promote local tissue anoxia, cerebral, myocardial or other organ infarctions and/or deep vein thrombosis. Aggregating phenomena also can occur due to stasis in a blood vessel brought about by surgical procedures. The aggregating tendency of red blood cells is a manifestation of attraction between their surfaces which can also be detected as an increase in sliding friction, if the cells are forced to move past each other slowly. These are the conditions easily attained in flow in the microcirculatory vessels: arterioles, capillaries, the venules, especially in the ever dividing channels of the arteriolar circuit and ever converging channels of the venular circuit. Typical inner diameters of these vessels range from 200 .mu.m to 8 .mu.m, and probably most of the arterio-venous blood pressure drop occurs in arterioles and capillaries having diameters less than 50 .mu.m. Accordingly, it would be highly desirable to provide a determination of a patient's apparent blood viscosity under conditions relevant to flow in his microcirculatory vessels which is accurate, reproducible and simple to operate so that a patient can be appropriately diagnosed and treated in order to minimize or avoid the physiological risks of cellular aggregation.
Many presently available techniques for measuring blood viscosity are really aimed at measuring the end-point of surface-induced blood coagulation, the gel point, whereas in the described invention complications of coagulation are intentionally avoided. For example, U.S. Pat. No. 3,587,295 discloses a procedure for measuring the coagulation characteristics of blood by subjecting the blood to mechanical energy and measuring the intensity of the energy transmitted to the blood which then is correlated with the coagulation characteristics of the blood. U.S. Pat. No. 3,053,078 also utilizes an indirect methods whereby a rotatable means is inserted into the blood and rotated at a constant velocity and the resistance to rotation then is measured and correlated with the coagulation characteristics of the blood. U.S. Pat. No. 3,911,728 discloses a process for measuring blood viscosity by placing a blood sample and a confined gas in a tube having a narrow cross-section and reciprocally moving the blood through the narrow cross-section. The gas pressure variations due to compression of the gas are measured and then correlated with viscosity. Other indirect means for measuring physical characteristics of blood are shown in U.S. Pat. Nos. 3,918,908; 3,967,934; 4,187,462 and 4,202,204. Since the means for measuring blood viscosity as disclosed in the cited patents are indirect, errors are introduced which render the results for less reliable than could be obtained with a direct blood viscosity measurement.
When the viscosity of anticoagulated blood is determined by conventional capillary viscometers, cone-and-plate viscometers, or cylindrical viscometers, the flow rates or shear rates are usually so high that the sliding friction and aggregating effects are obliterated, and the viscosity of the blood determined under such conditions appears to be both Newtonian (independent of flow rate) and dependent only on volume percent red cells (hematocrit). Consequently the clinician has often relied on hematocrit reading as a guide to probable blood viscosity level, unaware of the fact that macromolecular plasma concentrations, especially of fibrinogen, can greatly increase the level of apparent viscosity that will be relevant in microcirculating flows.